Secret signaling system



Filed June 3, -1944 2 Sheets-Sheet l www July 23, 1946. c. E. ATKlNs Y SECRET SGNALING SYSTEM 2 t e e h s S t e nv .n s 2 Filled June 5, 1944 INVENTOR 642.4 fan/Ha T/f//vs wma ESI Ehm Patented July 23, 1946 SECRET SIGNALING SYSTEM Carl Edward Atkins, Elgin, Ill., assignor to Tung- Sol YLamp Works, Inc., Newark, N. lJ., `a corporation of Delaware Application June 3, 1944, Serial No. '538,667

Claims. (Cl. 1791.5)

My present invention relates to secret stationto-station communication-systems, of the general type disclosed in my copending applications Serial No. 443,898, led May 21, 1942, and Serial No. 504,679, iiled October 2, 1943, wherein the signal originating at one station is used to scramble the signal transmitted from the other station. In the system of application Serial No. 504,679, intermodulation -is effected by simultaneously keying the equipment a't the two stations. A monotone of the same frequency Iis yinjected in such phase relationship that there is :no change in amplitude when the keys at both stations are simultaneously depressed but only a change of phase. The present system differs from the system above briefly described and from the system of application Serial No. 443,898 in that when the .keys at the two stations are simultaneously depressed no signal is transmitted from one lstation to the other. In the present system the signal energy from one transmitter serves to reverse the background conditions for the other. Thus during a period of silence a telegraphic symbol may be transmitted while during the subsequent period, which may be a millisecond, several seconds or even a minute, the appearance of a .signal might mean that no character at all was being sent but that an alternate type of background was being used.

For an understanding of the present invention reference may be had to theaccompanying 4drawings, 'of which Fig. 1 is a simplified block diagram of equipment at two communicating stations for operation of the new system;

Fig. 2 is a circuit diagram of the complete equipment suitable for use at 'each station and representing the preferred embodiment of `the invention; and

Fig. 3 is a graph explanatory of therelation between charging potential and gateY output .in the various networks of Fig. .2.

'Ihe new system and its operation will 'i'lrst be described by reference to the simple diagram Iof Fig. 1. As shown .in liig. 1, the essential fequipment at station vA comprises a .receiving v,gate 2a, detector 4a, transmitting gate 6a, a delay system Ba, and `a key 'lila for introduction of the signal, the output from those various instrumentalities being delivered in the directions indicated by the respective arrows. similar equipment, namely, receiving gate 2b, a detector 4b, a transmitting gate 6b, a delay system 812, and a .key lb. Each gate Yhas two input terminals and one :output terminal. Its yfunction is to .pass a signal when activated by vStation B .is provided with by and` delivered through the delay system 8a and' .hence Jboth input terminals lof the gate 2a beingy activatedmo signal isjpassedto `detector 4a. Y Sim- 'Y ilarly, when key vMib onlyis depressed, the signalV V is passed by transmitting :gate 6b, radiated from v2 either of two sources butY not when activated by bothsources simultaneously. Thus whenkey Illa is depressed vand if there is vnooutput fromthe detector 4a, transmitting gate 6a will'operate to pass a signal, .but if the input terminals of gate 6a are Asimultaneously activated .from the signal source by lthe -key lila and from the detector 4a, no signal is'v passed by the gate. Conversely, if key Ia is not depressed but a lsignal is impressed upon gate a'from detector 4a, the gate passes the signal. The ysame is true of transmitting gate 6b and of receiving gates 2a and 2b. These gaies are varieties of electronic switches and ywill be more fully described hereinafter .in connection with Fig. 2. The equipment at each stationY will ,include also, of course, the necessary radioV frequency apparatus for the transmission and radiation of the signal passed by the transmitting gate and for reception and demodulation of the energy radiated from the other station before delivery of the ysignal to the rreceiving gate. These elements kof the 4system Vhave been omitted from Fig. 1 for simplicity .but are included in the circuit drawing of.Fig.,2.

Inoperation of the system of Fig. 1, if key i-Ua is depressed, transmitting gat-e -Ba is activated and a signal is sent to station B, where, after reception and demodulation, receiving gate 2b is acti- Vated and passes the signal on toI detector 4b. In the output of detector 4b the signal is Aavail- ,able fas useful intelligence and is also used to actuate ytransmitting gate 6b which causesthe signal to be returned to station A for application to vone input terminal of receiving gate 2a. At the same time .that the depression o-fkey lilla started the signal onits wayto station B and back to station A, Vit also :started a similar signal,

.a pulse of exactly the same time duration, through Y l .the Ylocal delay system '8a.Y Delay system 8a, yas

`.more fully described in connection with Fig; l2,

isconstructed to simulate @the time characteristics of the equipment at station B and of the intervening space.. .'Ihus Ythe yreturning signal-arrives ,atione of the input terminals of receiving gate 2a simultaneously with the arrival at the other input terminal of asimilar signal locally retained station B, received at :stati-on A, 'passed by. receiving gate i211, detected at station A, -passed by transmitting Igate iifartransmitted back tostation,

yB 'and L"blocked'atreceivingtgate12b :by the duplicate signal retained and delayed by delay system 8b. If key Ita is depressed and subsequently key ich is depressed at the moment when the signal from key Ia rst reaches the portals of transmitting gate 6b, that is after transmission from station A, reception and detection at station B, then transmitting gate 6b will not operate and nothing will be transmitted to station A. However the depression of key IDb will be detected at station A because receiving gate 2a, being activated only by the delayed signal through system 8a, will pass energy to operate detector 4a. If key Ilia is still depressed, then after detection by detector 4m, transmitting gate 6a will cease to operate. No signal will be radiated to station B but receiving gate 2b will operate because of activation through delay system 8b. Thus if both keys are depressed for a substantial length of time no signal is sent from one station to the other but the fact of the depression of the key of one station is made apparent by the operation of the detector at the other station. If after both keys have been depressed and no signal is being transmitted from one station to the other, as above described, one of the keys is lifted, a signal will be sent from that station and after a suitable delay a signal will follow from the other station. If both keys are worked the keying at the two stations will be superimposed and each signal will be subjected to a more or less capricious scrambling. Thus if one key is held down for a sustained period of time, periods of' depression of the other key correspond to periods of no signal transmission between stations. Thus the holding down of one key has the effect of reversal of background for the other, so far as concerns an interceptor of the transmitted radiation.

Specific apparatus functioning as above described in connection with the simple block diagram of Fig. 1 is shown in Fig. 2 to which referencemay now be had. In Fig. 2 a suitable receiving mechanism I2 is shown Within the dashed line enclosure as including an antenna, a wave trap tuned to the transmitting frequency of the same station and conventional radio frequency amplifier, demodulator and audio frequency amplifier circuits. The output of the receiving mechanism I2 is fed to a rectifier I4 which differs from the conventional half wave diode rectifier circuit `in the inclusion of a series resistor I6. The function' of resistor I6 will become apparent herinafter.

rIhe output from rectifier I4 provides the D. C. voltage needed for activation of the receiving gate 2. As shown, gate 2 includes a pair of tubes I8 and I9 such as the 6 SAlGT. The cathodes of the tubes are connected through a resistor 2n having an adjustable center grounded tap. The inner grid of tube I8 is connected to the output circuit of rectifier I4 and the inner grid of tube I9 is connected to delay system 8. The outer grids of tubes I8 and I9 are excited by radio frequency voltages from an oscillator circuit through a tuned transformer with a center tapped arrangement. The oscillator circuit, here shown as of the Colpitts type, and the transformer are indicated by the reference number 22. The plates of tubes I8 and I9 are parallel connected to a tuned-circuit comprising a variable capacity 23 connected across the primary of a radio frequency transformer 24. The tap on resistorA is so adjusted that when the inner gridsof tubes I8 and .I9 are at ground potential, substantially no energy appears in the output circuit of the gate. When a negative potential is applied to the inner grid of either tube I3 or I9, that tube is blocked,

the balance of the gate is upset and energy is passed. When the inner grids of both tubes are negative, both tubes are blocked and the gate passes no energy. The gate 2 is thus a kind of three position switch which is off when the inner grids of both tubes are at ground potential, can be turned on by a suitablenegative voltage on the inner grid of either oneof the tubes, and can be turned off again if' the corresponding grid of the other tube is likewise made negative.

In the particular embodiment of the invention illustrated in Fig. 2, receiving gate 2, as heretofore indicated, is excited by radio frequency energy applied to the outer grids of tubes I8 and I9. Audio frequency energy could be employed for this purpose but radio frequency energy is preferred in order that transmitting gate 6 can be actuated by a rectier system containing little or no time delay.

Energy passed by the transformer 24 is rectified by rectier 26 and appears across load network 28. As previously indicated network 28 need introduce little or no time delay and its rectified potential is almost immediately applied to a grid of one tube of the transmitting gate 6. The elements 23, 24, 26 and 28 comprise the detector 4 corresponding to the units 4a and 4b of Fig. 1. As indicated in Fig. 1, the output of the detector may yield the useful signal, for example, the voltage across network 28 can be directly utilized as a signal or indicator operating means. Preferably, however, this Voltage is used indirectly to operate a separate detector unit 30 as hereinafter described.

The transmitting gate 6 is substantially similar to gate 2 except that it is arranged for excitation by audio frequency rather than radio frequency energy. As shown, gate 6 includes two tubes 3I and 32 having their cathodes connected by a resistor having a variable center tapped ground connection, their anodes connected in parallel to the primary of a'transformer 34 and their outer control grids connected for excitation across a center tapped secondary of transformer 36 connected to a source of audio frequency energy. The inner control grid of tube 3l is connected to the network 28 of detector 4 and the inner control grid of tube 32 is connected through a network 40 simulating the characteristics of network 28; and a resistor 4I with the key I0. The key Il), when closed applies a negative potential to the grid of tube 33 su'icient to block operation of the tube.

The detector unit 30 includes a high y. triode f- 42 the grid of which is excited vby an audio frequency tone and the cathode of which is connected to the cathode of tube 3| of gate 6. Normally the positive potential on the cathode of tube 42, due to the space current of gate tube 3'I so blocks the tube 42 that the audio frequency tone is not heard in the ear phones 44 connected to the plate of tube 42 through the' blocking condenser 45. When gate tube 3I is blocked, however, by voltages appearing across network 28 of detector 5, tube 42 becomes operative and a tone is heard in the head phones 44. This corresponds to the keying action at the cooperative station when the system is operating properly.

In the particular embodiment'of rthe invention illustrated in Fig. 2, the output of transmitting gate 6 is used to amplitude modulate a'transmitter 46. Instead of amplitude modulation,

phase or frequency modulation could bel employed at one or both stations. 21;. 'I'he delay system '.8 which". comprisesl aseries of gates and lters will now be described." The :first gate includes a. .pair of tubes 48 and 49 VAhaving their anodes connected in parallel through Va load impedance 50 to a source of positive poten tial and their cathodes connected to ground through adjustable resistors I and 452. The outer grids of tubes 48 and -49 are connected ,for excitation to the transformer 36. The vinner grid of tube 48 is connected through network 40 and resistor 4I with key lil while the inner grid of tube 49 is grounded. Thus 'when key I- is up, both inner grids are at ground potential and the gate comprising tubes 48 and 49 does not pass energy, but when key I'U is depressed, tube '48 is blocked by the application of negative voltages to its cathode, the gate becomes unbalanced and a signal appears across impedance 5U. 'This signal is fed through a delay filter 54 designed and yadjusted to simulate the delay occasioned by transmission of the signal through the lspace from one station to the other. From the lter 54, the ysignal is fed through a rectiiier network having time characteristics simulating lthose of detector I4 at the other station. Output from rectier 56 unbalances a second gate of the delay system 8 comprising the tubes il and 58 causing the signal to appear across a load impedance 6B and to be fed to a second delay lter 6'2. Filter '62, like -lter 54, is designed to simulate the delay occasioned by transmission of the signal through space from one station to the other. LFinally, the output from filter 6'2 is applied to a rectifier '64 having time characteristics similar to rectier i4 of the same station. The rectified potential across the network 66 in the output o1" rectier 64 is applied to the inner grid of tube I9 of receiving Igate 2.

When two stations are each equipped with the instrumentalities of Fig. 2 and when the various networks thereof are properly adjusted, the lsystem will operate as previously described in connection with Fig. 1. In order to insure that the concurrent depression of the keys at the two stations causes an absence of signalling between the stations, precise timing is essential, that is the charge rates and also the discharge rates of all important time delay networks must be identical. This is due to the fact that the depression of a key while the system is quiescent calls for the charging of all the cascade networks and the concurrent and synchronous charging rof similar networks in the delay lsystem 8, while when the system is operative due to the depression of one key, the subsequent depression of the key at the other station serves to discharge all the signal channel networks while matching this action timewise with a charging procedure in the local delay system 8. This requirement explains the special form of rectifier units used in the system. The series resistors, such as resistor I6 in unit I 4, retards the charging rate for the shunt condenser in the output network of the rectifier so that the same amount of time will .be required for charging as for discharging. It should be noted that when the exciting input voltage of a rectier unit disappears, the rectilinear character of 'the diode of the rectier serves to isolate the output network from the rest of the system so that its discharge rate is for the most part a function of its own constants.

Since the gates are voltage operated devices, any consideration of the time delay involved in operating them must also consider the magnitude of the various negative charging potentials and .6 networkcondensers become charged at maximum; YIn 3 the krelation lbetween `charging potential andA gate output is graphically representedi If the maximum negative charging potential for the inner grid Vof one lof the tubes of a gate is .repre sented by E1 lon Fig. '3, then from the graph it isapparent that the gate output will reach its maximum prior to complete charging of the gate grid and that minor variations in the charging potential will have no eifect upon the gate output. On the other ihandl'with-a maximumcharging potential represented by the lineqEe, .the gate out-put will not reach-its maximum .value and variations in the charging potential result in variations in gate output; In order for the system as a whole to operate ias hereinbeore de'- scribed, it is thus necessary to correlate` the mag.- .nitudes of the resistors and condensers ofthe various networks and the charging potential, such as E1, E2 or Es, so asto insure 'equalftime delays during charging and discharging of the grids 'of the various gates.

The invention has now beenV described in connection with one -specic embodiment thereof. Obviously the invention as diagrammatically il.- lustrated in Fig. .l -could be embodied in equipment differing from that specifically illustrated v in Fig. 2, andv various changes inthe circuits of Fig. 2 will -occur to those skilled in the art.` .For example, although the blocking `of a gatetube has been show-n and described asv beingefi'ected by the application of a negative voltage `to the inner grid of the tube, the'tube could be blocked by other means, as for example by groundingof the outer grid. Although the particular hal'f Y wave rectifier `circuits with ltheir accompanying -time delay element heretofore described are `preferably employed inthe system, full wave rectiiiers involving 'substantial -reduction in time delay `could be employed if desired. Oth'er alternatives within the spirit of `the invention as dened in to those skilled means `and the 'terminal o'f said receivingfgate :for introducing a time delay between operation .of the .key means Yand `activation of the terminal ofV theV receiving gate, 'a 'detector connec'tedfI-bei tween-said receiving. .gate andthe other' input terminal of said transmitting gate', 'and .means at each station for transmitting energyY passed by :the transmitting gate to the other station Afor .actuation of the other input .terminal of the `receiving gate at that station whereby 'the.rletector at each station can be made torespon'd -only to -signals initiated by the key means at the distant station. ii

Y2. A station-to-st-ation 'secret signaling system comprising interconnected receiving andftrans-A mitting means at each station, signal introducing means at each station, detecting means at each station interposed between said receiving and transmitting means, means at each station therefore the absolute value to which any of the for creating a delayed replica of the locally introduced signal, means for suppressing the transmission .of a signal when the signal introducing means and the detecting means are simultaneously operated, and means jointly controlled by said means for creating a delayed replica of the local signal and by said receiving means for suppressing operation of the detecting means when the energy passed by said receiving means results from the retransmission from the other station of the signal previously introduced at the same station whereby no signal is transmitted from one station to the other during simultaneous operation of the signal lintroducing means at lboth stations.

3. The secret signaling system according to claim 2 wherein said means for suppressing the transmission of a signal when the signal introducing means and the detecting means are simultaneously operated comprises a pair of tubes each having a cathode, an anode and at least two grids, a center grounded resistor connected between said cathodes, an output circuit connected to said transmitting means and to which the anodes of said tubes are parallel connected, a source of energy connected to one grid of each tube for applying alternating voltages thereto in opposite phase, a second grid of one tube being connected to said signal introducing means and a second grid of the other tube being connected to said detecting means whereby the application of negative voltage to the second grid of one tube only resulting either from the operation of the signal introducing means or of the detecting means blocks one tube, upsets the balance of the tubes and permits passage of energy to the transmitting means, whereas simultaneous application of negative voltage to the second grids of both tubes blocks both tubes and suppresses the passage of energy to the transmitting means.

4. The secret signaling system according to claim 2 wherein each of said receiving means includes a rectier circuit and wherein each of said means for creating a delayed replica of the signal includes a circuit simulating the time constants of the rectifier circuit at the same station, a circuit simulating the time constants of the rectiiier circuit at the other station, and circuits simulating the time constants of the space intervening between the stations, each of said circuits having charging periods equal to their discharging periods.

5. In a secret station-to-station signaling system, an electronic switch having two input terminals and one output terminal and arranged to pass energy when a negative voltage is impressed on either input terminal and to be blocked when negative voltage is impressed on neither or both input terminals, a source of negative Voltage, a network between said source and one of said input terminals and key means between said source and said network, said network having equal charging and discharging periods, a detecting circuit connected to said other input terminal, and transmitting means connected to the output terminal of said electronic switch for transmission of a signal when said key is operated and the detecting circuit is not energized or when said key is in open circuit position and the detecting circuit is passing energy.

fea

6. The signaling system according to. claim 2 wherein said receiving means includes a rectier circuit comprising a diode, an input circuit connected to the cathode of said diode comprising a condenser and resistor connected in series and a resistor having one end connected to said condenser and to said first resistor and its other end connected to a source of low positive potential and an output circuit connectedr to the anode of said diode comprising a resistor anda condenser connected in parallel, the various resistors and condensers being so proportioned that the time rate of charging of said output network, upon application of energy to the condenser in the input circuit equals the time rate of discharge of the output network upon cessation of the application of energy to the input condenser.

7. In a secret station-to-station signaling system equipment at each station comprising receiving means, an electronic switch, a detector, a second electronic switch and transmitting means all connected in a chain in the given order, key operated means connected to both of said electronic switches, and a delay circuit interposed between said key operated means and the first mentioned switch, said rst mentioned switch operating to pass energy to said detector only when activated either by said key means through said delay circuit or by said receiving means and said second mentioned switch operating to pass energy to said transmitting means only when activated either by said key means or by said detector. Y

8. Station equipment in a secret signaling system according to claim 7 wherein said delay circuit of one station includes networks simulating the time constants of the chain of the other station, of the intervening space between stations and of the receiving means at the same station whereby energy passed by said second switch as a result of operation of said key means, transmitted to the other station, passed through the equipment at the other station, retransmitted back to the first station and received by the receiving means at the rst station Iarrives at said first mentioned switch simultaneously with energy passed by said delay circuit as a result of the same operation of the key means.

9. The method of secret signaling between two stations which comprises blocking the transmission of a signal between stations during such periods as keys are simultaneously depressed at both stations and during such periods detecting at one station the signal at the other station by reference to the signal at the same station.

10. The method of secret signaling between two stations which comprises passing modulating energy to a transmitter at one station for transmission to the other station upon operation of the key at the same station, detecting the modulation at the other station and passing modulating energy to the transmitter at the other station in response to such detection for retransmission to the rst station, blocking detection of such modulation at the rst station by energy controlled by operation of the key and delayed at the rst station, while blocking transmission of modulating energy to the transmitter of a station during concurrent detection and key operation of the same station.

CARL EDWARD ATKINS. 

